General valve application requirements dictate that motorized valve actuators or operators must have provision for manual operation in case of power failure or in the case of initial installation and set-up when power may not be available. Safety requirements dictate that the power and manual drives operate independently so as to preclude movement of the manual input (handwheel, lever, etc.) during power operation. Traditionally, the requirement for independent manual operation has been satisfied by placing a clutching device within or attached to the actuator power train. The function of the clutching device is to nate drive (manual or power).
Engagement or disengagement of the clutching device can be accomplished by several means including: (i) depressing a lever to move a clutch element; (ii) pushing or pulling a handwheel into engagement; and (iii) rotating a handwheel which causes clutch elements to engage.
All of the above provide for totally independent drive, either manual or motor, i.e., the drive arrangement is either motor or manual, the drives are never engaged simultaneously.
Other types of clutches may be used which provide for single direction power transmission, i.e., clockwise or counterclockwise torque may be transmitted via a handwheel through the clutch to the power train but may not be transmitted from the power train through the clutch to the handwheel. Clutch arrangements of this type have the inherent disadvantage of back-driving the power element unless the power element is disengaged by alternate means.
Also known in the art is the valve actuator disclosed in U.S. Pat. No. 4,261,224 to Sulzer. There, a valve actuator is operable alternatively by a motor or manually. The motor is coupled through a special one-way coupling to the sun-gear drive of the planetary assembly which permits the motor to drive the sun gear in either direction upon rotation of the motor but prevents rotation of the sun gear when the motor is arrested. A handwheel is coupled to the ring-gear drive of the planetary assembly through a worm and pinion so that the handwheel may then drive the ring gear under manual operation, but the ring gear is retained against rotation by the worm pinion when the handwheel is idle.